Improvements over the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids has been the subject of important research and development in the petroleum industry for many years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin (PAO) synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature i.e.,improved viscosity index, while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants lower friction and hence increase mechanical efficiency over a wider range of operating conditions than mineral oil lubricants.
Notwithstanding their generally superior properties, PAO lubricants are often formulated with additives, or an additive package, to enhance those properties for specific applications. The more commonly used additives include oxidation inhibitors, rust inhibitors, metal passivators, antiwear agents, extreme pressure additives, pour point depressants, detergent-dispersants, viscosity index (VI) improvers, foam inhibitors and the like. This aspect of the lubricant arts is specifically described in Kirk-Othmer "Encyclopedia of Chemical Technology", 3rd edition, Vol. 14, pp. 477-526, incorporated herein by reference.
Lube additive packages are complex and costly materials. Therefore, a significantly superior new lubricant is one in which the properties conferred on lube mixture by known additives are intrinsic to the structure of the new lubricant molecule, obviating or at least reducing the need for additives. Consequently, improvements in lubricant technology pursued by artisans in the field flow from both new additive development addressed to deficiencies in lubricant oligomers and new oligomer development for inherently better properties to displace additives. Increasing the aromaticity of lube mixtures is known to improve thermal stability and antiwear properties. However, in view of the paraffinic nature of PAO lubricants aromatic additives to PAO are often confronted with problems of solubility that limit their usefulness or require the use of costly dispersants.
Alkylated aromatics are known in the prior art as lubricant additives for their antiwear properties, thermal and oxidative stability as disclosed in U.S. Pat. Nos. 4,211,665, 4,238,343, 4,604,491 and 4,714,7944. Antiwear properties of alkylnaphthalene lubricating fluids are presented in Khimiya i Tekhnologiya Topliv i Masel, No. 8, pp. 28-29, August, 1986.
Recently, novel lubricant compositions (referred to herein as HVI-PAO) comprising polyalpha-olefins and methods for their preparation employing as catalyst reduced chromium on a silica support have been disclosed in U.S. patent applications serial number 210,434 and 210,435 filed June 23, 1988, incorporated herein by reference. The process comprises contacting C.sub.6 -C.sub.20 1-alkene feedstock with reduced valence state chromium oxide catalyst on porous silica support under oligomerizing conditions in an oligomerization zone whereby high viscosity, high VI liquid hydrocarbon lubricant is produced having branch ratios less than 0.19 and pour point below -15.degree. C. Lubricants produced by the process cover the full range of lubricant viscosities and exhibit a remarkably high VI and low pour point even at high viscosity. The molecular structure of HVI-PAO is novel, comprising the product of an essentially regular head to tail polymerization of alpha-olefin and providing an oligomer with large pendant alkyl groups on the recurring polymeric unit.
Dehydrocyclization is a well known reaction in the organic chemical arts for the conversion of linear and branched alkanes to aromatic compounds, as described in Royals "Advanced Organic Chemistry", Prentice-Hall, Inc., pp 145-147, incorporated herein by reference. The reaction is carried out typically by catalysis with dehydrogenation catalysts and proceeds through ring closure and dehydrogenation to provide an aromatic structure. The dehydrocyclization reaction can be characterized as a type of aromatization reaction and the terms are used interchangeably herein.
It is an object of the present invention to provide novel lubricant compositions having improved thermal stability or antiwear resistance and high viscosity index.
Yet another object of the present invention is to provide novel lubricant compositions comprising HVI-PAO incorporating aromatic branches in the oligomer structure.
A further object of the present invention is to provide novel HVI-PAO compositions having enhanced aromaticity and high VI.
Another object of the present invention is to provide a process for producing HVI-PAO containing increased aromaticity without significantly degrading viscosity or VI.